Spermatogenesis of Zaprionus indianus and Zaprionus sepsoides (Diptera,Drosophilidae): Cytochemical, structural and ultrastructural characterization

Letícia do Nascimento Andrade de Almeida Rego, Rosana Silistino-Souza,

Maria Tercília Vilela de Azeredo-Oliveira and Lilian Madi-Ravazzi

Departamento de Biologia, Instituto de Biociências, Letras e Ciências Exatas,

Universidade Estadual Paulista “Julio Mesquita Filho”, São José do Rio Preto, SP, Brazil.

Abstract

Zaprionus indianus is a drosophilid native to the Afrotropical region that has colonized South America and exhibits awide geographical distribution. In contrast, Z. sepsoides is restricted to certain African regions. The two species differin the size of their testes, which are larger in Z. indianus than in Z. sepsoides. To better understand the biology andthe degree of differentiation of these species, the current study evaluated spermatogenesis in males of different agesby conventional staining techniques and ultrastructural analysis. Spermatogenesis and the ultrastructure of sperma-tozoa were similar in the two species, and the diploid number was confirmed to be 2n = 12. A greater number of sper-matozoa were observed in young Z. indianus (1-3 days old) compared to Z. sepsoides males, which showed a higherfrequency of cells at the early stages of spermatogenesis. The head of the sperm was strongly marked by silver stain-ing, lacto-acetic orcein and the Feulgen reaction; the P.A.S. reaction revealed glycogen granules in the testes of bothspecies. Both species presented similar arrangement of microtubules (9+9+2), two mitochondrial derivatives of dif-ferent size and 64 spermatozoa per bundle. Such similarity within the genus Zaprionus with other species ofDrosophila, indicates that these structures are conserved in the family Drosophilidae. The differences observed thenumber and frequency of sperm cells in the early stages of spermatogenesis, between the young males of Z.indianus and Z. sepsoides, are features that may interfere with reproductive success and be related to the invasivepotential of Z. indianus.

Keywords: Zaprionus indianus, Zaprionus sepsoides, spermatogenesis, cytochemistry, ultrastructure, meiosis.

Received: July 5, 2012; Accepted: October 15, 2012.

Introduction

The genus Zaprionus is divided into two biogeo-

graphically separate subgenera: the subgenus Anaprionus

Okada, with approximately 10 species in the Eastern re-

gion, and the Zaprionus subgenera, with approximately 50

species in the Afrotropical region (Okada and Carson,

1983; Yassin et al., 2008). Phylogenetically, Zaprionus is

very close to the genus Drosophila (Yassin et al., 2007,

2008, 2010; Yassin and David, 2010).

The species Zaprionus indianus (Gupta, 1970) origi-

nated in Africa was first identified in Brazil in 1999 and is

considered an invasive species that has infested fig planta-

tions (Vilela, 1999; Tidon et al., 2003). After 1999, Z.

indianus has been detected in different regions of Brazil

(Toni et al., 2001, Castro and Valente, 2001; Santos et al.,

2003; Kato et al., 2004; Mattos-Machado et al., 2005, Da-

vid et al., 2006), Uruguay (Goñi et al., 2001) and, in 2005,

Florida (USA) (Van der Linde et al., 2006) and Argentina

(Soto et al., 2006).

Studies examining the invasive potential of Z.

indianus, included its life cycle (Amoudi et al., 1991; Stein

et al., 2003), larval competition (Amoudi et al., 1993a) and

aspects of fitness (Amoudi et al., 1993b; Setta and Cara-

reto, 2005). Allozyme studies have indicated that plasticity

in the distribution of the allelic frequency at the Est-3 locus

may have also contributed to the success of this species in

spreading across the Americas (Galego and Carareto, 2007,

2010). However, few studies have examined the biology

and reproduction of Z. indianus (Araripe et al., 2004) or

other Zaprionus species (Yassin et al., 2007; Yassin and

David, 2010; Yassin et al., 2010).

Of the 49 species of the genus Zaprionus, only three

have become invasive: Z. indianus, Z. tuberculatus and Z.

ghesquierei (Chassagnard and Kraaijeveld, 1991). Z.

indianus is the most widespread species of the genus and

the ecologically most diverse of the Afrotropical droso-

philid fauna (Yassin et al., 2007, 2010; Yassin and David,

2010). The general habits of this species may be a major

Genetics and Molecular Biology, 36, 1, 50-60 (2013)

Copyright © 2013, Sociedade Brasileira de Genética. Printed in Brazil

www.sbg.org.br

Send correspondence to Lilian Madi-Ravazzi. Laboratório de Ge-nética, Ecologia e Evolução de Drosophila, Departamento de Biolo-gia, Instituto de Biociências, Letras e Ciências Exatas, Univer-sidade Estadual Paulista “Julio Mesquita Filho”, São José do RioPreto, SP, Brazil. E-mail: lilian@ibilce.unesp.br.

Research Article

factor in its successful occupancy of four continents (La-

chaise and Tsacas 1983, Schmitz et al., 2007). However,

other factors, including aspects of its reproduction, adapta-

tion and ecology, may also be involved in the invasion pro-

cess.

Male characteristics that influence the competitive

ability to mate and fertilization success can also interfere

with the fitness of the species. For example, sperm size is a

male trait that is predicted to be under strong sexual selec-

tion (Hosken et al., 2003, Snook, 2005; Scharer et al.,

2008), even though the selective advantage of different

sizes of sperm remains largely unclear (García-González

and Simmons, 2007; Amitin and Pitnick, 2007). The num-

ber of sperm and their developmental stages are also factors

that can contribute to the reproductive potential of the spe-

cies.

The present study aims to expand on previous studies

on the biology of Z. indianus and the characteristics that

possibly influence its adaptation to various environments,

as reflected by its successful invasion of different conti-

nents. More specifically, this paper analyzes spermato-

genesis and sperm ultrastructure of a geographical strain of

Z. indianus and of Z. sepsoides (Duda, 1939). Z. sepsoides

is restricted to certain regions of East Africa and differs

from Z. indianus in testes size and sperm. Furthermore, it is

not considered an invasive species.

Material and Methods

Origin of insects

A strain of Z. indianus from Ubatuba, SP, Brazil, and

a strain of Z. sepsoides originating from the Congo region

of Africa were used in the present work. These strains were

maintained at � 25 °C in standard culture medium: banana

and yeast (Saccharomyces cerevisiae).

Measurements of testes

Pairs of testes were dissected from 12 eight-day-old

adult males of each species for obtaining linear measure-

ments. For such measurements, the spiral testes from Z.

indianus were first uncoiled.

Preparation of slides and cytochemical techniques

In this analysis, the testes were removed from males

of different ages (1-8 days of life) from both species. The

testes were stained using lacto-acetic orcein following De

Vaio et al. (1985) and impregnated with silver nitrate fol-

lowing Howell and Black (1980), with modifications. After

the usual preparation of the slides, the procedure for the

Feulgen reaction was followed according to Mello and

Vidal (1980), with modifications, for both species.

The structural and ultrastructural analyses were per-

formed according to Cotta-Pereira et al. (1976), with modi-

fications. The testes of adult Z. indianus and Z. sepsoides

males were removed at different developmental stages (1,

3, 5 and 8 days old) and fixed in a solution of 4% para-

formaldehyde and 2.5% glutaraldehyde in 0.2 M phosphate

buffer, pH 7.4. After initial fixation for 4 h at 4 °C, the tes-

tes were washed three times in 0.1 M phosphate buffer, pH

7.4, for 15 min each. Next, the material was post-fixed in a

1% osmium tetroxide solution for 2 h in a dark, sealed bot-

tle in the refrigerator. After this procedure, the material was

washed three times in distilled water for 5 min each and

then dehydrated in a series of increasingly concentrated ac-

etone solutions (30%, 50%, 70% and 90% acetone), with

15 min per solution, followed by dehydration in 95% and

100% acetone three times for 15 min each. After dehydra-

tion, the material was embedded in a 1:1 mixture of resin

(Araldite) and 100% acetone for infiltration overnight at

room temperature. The next day, the material was removed

from the resin-acetone solution, placed in resin and placed

in a 37 °C oven for 2 h. The testes were then removed from

the bottles containing the pure resin, and the embedding

process was initiated in the mold, which was placed in a

60 °C oven for 72 h for polymerization.

Semi-thin, 0.5 �m sections of the testes were stained

with 1% toluidine blue and analyzed using a light micro-

scope. Ultra-thin, 70 nm sections were contrasted with ura-

nyl acetate and lead citrate and analyzed by transmission

electron microscopy.

For periodic acid-Schiff (P.A.S.) staining, the slides

were first prepared for structural analysis for the two spe-

cies, as follows (Mello and Vidal, 1980, with modifica-

tions). A solution of 0.5% periodic acid was applied to the

semi-thin, 0.5 �m sections, which were then placed in a

60 °C oven for 14 min. Next, the sections were washed in

distilled running water for 5 min. A Schiff reagent was

added to slides, which were protected from light and were

placed in a 60 °C oven for 45 min. Following the washing

of each sample in 3 distilled water baths for 5 min each; the

slides were air-dried and mounted with nail polish.

The material prepared by means of conventional

cytochemical and staining techniques was analyzed using a

light microscope (Olympus BX40) and Axiovision LE dig-

ital imaging software, Version 4.8 for Windows. The mate-

rial prepared using histological techniques was analyzed

and photographed using a JEOL 1011 transmission elec-

tron microscope operated at 80 kV.

Results

Testes morphology of Z. indianus and Z. sepsoidesmales

The testes of Z. indianus and Z. sepsoides are yellow

in color (Figures 1A and 1C, respectively). In Z. indianus,

the testes consist of two coiled tubes of approximately

5 mm in length each (Figure 1A), whereas in Z. sepsoides,

the testes are two kidney-shaped tubes of approximately

2 mm in length each (Figure 1C).

Rego et al. 51

Lacto-acetic orcein staining and Feulgen reaction

In the apical part of the testes, a large number of di-

viding cells was observed. During prophase I, the nuclei of

the primary spermatocytes in the diplotene and diakinesis

stages exhibited chromosomes that were arranged in a cir-

cular ring and were associated with each other. The sex

chromosomes were separate from this ring, as indicated by

arrows in Figures 2A and 2J.

The metaphase stage of meiosis I revealed six pairs of

bivalent chromosomes (2n = 12) in the nucleus in a circular

arrangement (Figures 2B-C, 2E-F and 2K and 2L).

At the end of telophase I, the formation of two nuclei

was observed (Figures 2D, 2G and 2M). During spermio-

genesis, the spermatids become elongated (Figures 2H and

2N) and organized into long bundles in Z. indianus (Figu-

re 2I) and short bundles in Z. sepsoides (Figure 2O). In Z.

sepsoides, these bundles stained strongly and revealed a

concentration of genetic material at their tips, correspond-

ing to the nuclei of spermatozoids during individualization

(Figure 2O). This strong staining indicated the presence of

aldehyde groups, based on the color produced by the reac-

tive Schiff stain.

Mainly primary spermatocytes were observed in the

apical part of the testes of Z. indianus and Z. sepsoides,

whereas in other parts of the testes, secondary spermatocytes,

spermatids and spermatozoa were noted (Figures 1B and 1D).

Impregnation by AgNOR

Cells in various stages of spermatogenesis were ob-

served by impregnation with AgNOR. For both species,

spermatocytes in the early stages of prophase I were ob-

served, with nucleolar corpuscles scattered throughout the

cell nucleus (Figures 2P and 2S). Spermatids at later stages

of elongation showed no impregnation with AgNOR along

their entire length (Figures 2Q and 2T), whereas spermato-

zoa arranged into long bundles and strongly stained with

AgNOR in their extremities were frequently observed in Z.

indianus (Figure 2R). In contrast, in Z. sepsoides, sperm

bundles did not exhibit this staining pattern (Figure 2U).

Structural and ultrastructural analysis of the testesand spermatogenesis of Z. indianus and Z.sepsoides

The testes of Z. sepsoides and Z. indianus were ana-

lyzed at different stages of development (1, 3, 5 and 8 days

52 Spermatogenesis in Zaprionus sp.

Figure 1 - Testes and cells of Zaprionus indianus (A-B) and Zaprionus sepsoides (C-D). A and C. Testes seen under a stereo microscope. B and D.

Squash staining with lacto-aceto orcein showing the germinal center (GC) in the apex of the testes (ap), followed by cells in division (Cd) and spermato-

zoids (z) present in the central region of the testis. Scale bars: A and C (0.5 mm); B and D (10 �m).

old). In Z. sepsoides, the dividing cells (spermatocytes) and

elongating cells (spermatids) were more easily detected in

young males (1-3 days old), whereas in older ones sperma-

tozoids were noted more frequently. In contrast, in Z.

indianus, a greater abundance of sperm bundles was ob-

served in young males (1-3 days old), with only few cells in

the early stages of spermatogenesis.

In the testes of both species, diverse and unique cellu-

lar structures were found. The germinal center was the loca-

tion of a considerable number of germline stem cells, which

were identified at the apex of the testis in both Zaprionus

species. The testicular content could be divided into two

parts: germline stem cells at the apex of the testis and the re-

mainder of the testes filled with cells corresponding to

other stages of spermatogenesis. The testicle is coated by a

layer of cells and more internally by an envelope (Figures

1B, 1D, 3A, 3B, 4A, 4C and 5A). Strongly stained granules

were also observed, particularly in the testes of Z.

sepsoides. These granules may contain glycogen as in-

ferred from the results of the P.A.S. reaction on these testes

(Figure 3B).

The successive stages of spermatogenesis were not

visible in the testes of Z. indianus, perhaps due to their spi-

ral morphology and size, which hampered their proper

preparation as ultra-thin sections. Alternatively, the peak of

the initial stages may have occurred earlier in development,

for instance in the pupal stage. In Z. sepsoides, some stages

of spermatogenesis were observed, with dividing cells oc-

cupying the entire apical portion of the testis together with

the presence of many granules. Additionally, in regions

such as the testicular apex, many circular and elongated

spermatids were observed, and in other regions, bundles of

spermatozoa were distributed in all directions (Figu-

res 3A-B).

The differentiation of the spermatids in both species

within the cysts is synchronous (Figures 4A-B and 5A-B).

Another observation at ultrastructural level was the consis-

Rego et al. 53

Figure 2 - Histological and cytochemical staining results. Zaprionus indianus cells stained using lacto-aceto orcein (A-D) and subjected to the Feulgen

reaction (E-I). Zaprionus sepsoides cells stained using lacto-aceto orcein (J, K, M) and subjected to the Feulgen reaction (L, N, O). Testicular cells of

Zaprionus indianus (P-R) and Zaprionus sepsoides (S-U) after silver impregnation. A. Prophase I (diplotene / diakinesis phase), showing the chromo-

somes arranged in a circular ring and the sex chromosomes (arrows) as separate from this ring. B, C, E, F. Metaphase I of meiosis showing the six pairs of

bivalent chromosomes. D, G. Telophase I showing the genetic material in the center of each nucleus. H. Elongated spermatids I. Spermatozoids orga-

nized into long bundles. J. Prophase I (diplotene / diakinesis phase), showing the chromosomes arranged in a circular ring and the sex chromosomes (ar-

rows) as separate entities from this ring. K, L. Metaphase I of meiosis showing the six pairs of bivalent chromosomes. Scale bars: 5 �m.

tent number of 64 spermatozoids per bundle in both Z.

indianus (Figures 4A-B) and Z. sepsoides (Figures 5A-B).

The ultrastructures of spermatozoids in both Z.

indianus and Z. sepsoides revealed that the axonemes of

both species possess a 9+9+2 configuration, consisting of

one pair of central microtubules, nine double peripheral

microtubules formed by fibril A and fibril B, surrounded by

nine additional accessory microtubules, plus nine spokes

(Figures 4G-H and 5D-E, respectively). Additionally,

proximal to the axoneme, two mitochondrial derivatives of

different size were present (Figures 4B, 4D-G, and 5B-D)

in both species. The larger mitochondrial derivative con-

tained distinct paracrystalline material, particularly in Z.

indianus (Figures 4D-E).

Discussion

The rapid dispersion of Z. indianus across different

continents, and especially throughout Brazil, where this

species is considered a pest on fig plantations (Stein et al.,

54 Spermatogenesis in Zaprionus sp.

Figure 2 (cont.) - M. Telophase I showing the genetic material in the center of each nucleus. N. Elongated spermatids. O. Spermatozoid bundle present-

ing strong staining and a high concentration of chromosomal material at the tip of the bundles, this corresponding to the spermatozoid nuclei. (P-U).

Testicular cells of Zaprionus indianus after silver impregnation. P. Spermatocytes in the initial state of prophase I, showing nucleolar bodies spread

throughout the entire nucleus of each cell. Q. Spermatids in a more advanced stage of elongation not presenting specific impregnation along their entire

length; R. Spermatozoids organized into long bundles with extremities strongly stained by AgNOR (arrow). (S-U) Testicular cells of Zaprionus sepsoides

after silver impregnation. S. Spermatocytes in prophase I showing the nucleolar bodies of the cells revealed through silver impregnation; T. Spermatids in

a more advanced stage of elongation not presenting specific impregnation along their entire length; U. Spermatozoids not showing strong AgNOR stain-

ing in one of their extremities. Scale bars: 5 �m.

Figure 3 - Semi-thin histological sections of Zaprionus indianus (A) and Zaprionus sepsoides (B), showing spermatids (spmtd), followed by

spermatozoid bundles arranged transversely and longitudinally (z). Other structures are also present in the testes, such as the coating layer (cl), the enve-

lope (en) and glycogen granules (gr) revealed by P.A.S. reaction (note in detail of Figure B). Note these histological sections were 0.5 mm in length and

stained with 1% toluidine blue. Scale bars: 10 �m.

Rego et al. 55

Figure 4 - TEM micrographs of spermatozoids (flagellum) of Z. indianus. A. Transverse section of the testes showing various spermatozoid bundles or-

ganized within cysts. B. Transverse section of the testes showing a bundle containing 64 spermatozoids in detail. C. Transverse section of the testes show-

ing the coating layer (cl) and envelope (star). D-G. Transverse sections of a spermatozoid bundle, showing the tail region with axonemes (Ax) and mito-

chondrial derivatives of different size: larger mitochondrial derivatives (MD) in which the accumulation of paracrystalline material (p) is visible and

smaller mitochondrial derivatives (Md). H. details of a flagellum showing the 9+9+2 arrangement: one pair of central microtubules (CM), nine double pe-

ripheral microtubules (formed by fibril A and fibril B), surrounded by nine additional accessory microtubules (AM) and nine spokes (s). Scale: Figure A:

2000x; Figure B: 10000x; Figure C: 8000x; Figures D, E, F: 67000x; Figure G: 84000x; Figure H: 100000x.

2003), has spurred a growing number of studies on its biol-

ogy and history of invasion. A similar reason motivated the

present study on the spermatogenesis of this species in

comparison to Z. sepsoides, as the two species differ in their

ecological characteristics and testicular morphology. We

showed that the testes of Z. indianus are formed by two

coiled tubes of approximately 5 mm in length, whereas the

testes of Z. sepsoides consist of two tubes of kidney shaped

appearance of approximately 2 mm each. These measure-

ments confirm those of Yassin and David (2010) obtained

on testes of different species of Zaprionus. The inermis spe-

cies can be classified into two categories: those with small

testes, ranging from 1.0 to 2.0 mm in length (Z. sepsoides,

Z. inermis, Z. cercus, Z. kolodkinae and Z. tsacasi), and

those species with large testes, varying from 5.2 to 5.4 mm

in length (Z. indianus, Z. africanus and Z. taronus).

Araripe et al. (2004) showed that the anatomy and re-

productive physiology of Z. indianus differs from D.

melanogaster. For example, Z. indianus sperm are longer

(5 mm) than the sperm of D. melanogaster (1.8 mm). Joly

and Bressac (1994) measured the size of the sperm and tes-

tes of several drosophilid species and found that sperm size

is approximately 6.7 mm in Z. indianus and 0.78 mm in Z.

sepsoides and that the testicular sizes are 6.53 mm and 1.42

mm, respectively. These measurements correspond to the

testes measurements performed in this study.

The sperm bundles in Z. sepsoides were shorter than

in Z. indianus, reflecting its smaller size and thus smaller

testes. As frequently reported in the literature, testes are as

long as the sperm they produce (Lindsley and Tokuyasu,

1980; Pitnick and Markow, 1994; Snook, 2005; Scharer et

al., 2008).

The diploid number of Z. indianus and Z. sepsoides

was confirmed to be 2n = 12 chromosomes. In this study,

the AgNOR technique could not distinguish the nucleolar

organizer regions (NORs) in the meiotic chromosomes. In

prophase stage nuclei and in sperm, a NOR was intense pro-

tein synthesis in the anterior region, corresponding to the

spermatozoid heads. Although this marking has been ob-

served in studies of other organisms (Tavares and Aze-

redo-Oliveira, 1997; Tartarotti and Azeredo-Oliveira,

1999; Severi-Aguiar and Azeredo-Oliveira, 2005; Moriel-

le-Souza and Azeredo-Oliveira, 2008; Costa et al., 2008;

Peruquetti et al., 2008, 2010), few studies have examined

AgNOR markings in the genus Zaprionus. Gupta and Ku-

mar (1987) studied the association of polytene chromo-

somes with the nucleolus in four distinct Indian populations

of Zaprionus indianus and observed that different chromo-

somes, such as X chromosome and microchromosomes,

exhibit NOR activity.

In young Z. sepsoides males (1-3 days old), cells in

the early stages of spermatogenesis, such as spermatocytes

I and II, spermatids and a few spermatozoids, were ob-

56 Spermatogenesis in Zaprionus sp.

Figure 5 - TEM micrographs of spermatozoids (flagellum) of Z. sepsoides. A. Transverse section of the testes showing various spermatozoid bundles or-

ganized within cysts. B. Transverse section of the testes showing a bundle containing 64 spermatozoids in detail. C, D. Transverse section of a

spermatozoid bundle, showing the tail region with axonemes (Ax) and mitochondrial derivatives of different size: larger mitochondrial derivatives (MD)

and smaller mitochondrial derivatives (Md) are visible, beside a coating layer (cl) and an envelope (star) E. details of a flagellum, showing the 9+9+2 ar-

rangement: one pair of central microtubules (CM), nine double peripheral microtubules (note fibril A and fibril B), surrounded by nine additional acces-

sory microtubules (AM) and nine spokes (s). Note the accumulation of electron-dense material in the nine accessory microtubules and in the two central

microtubules. Scale: Figure A: 2000x; Figure B: 4000x; Figure C: 20000x; Figures D: 67000x; Figure E: 100000x.

served more frequently than in young Z. indianus males. In

contrast, the young Z. indianus males presented larger

numbers of spermatids and spermatozoa and few cells at

the initial stage of spermatogenesis. This finding suggests a

difference in sperm maturation dynamics between the two

species. In the genus Drosophila, spermatocytes enter in

the first meiotic division in the pupal stage (Cooper, 1950),

and the testes of most species of Drosophila are not fully

developed at hatching and, with certain variability, con-

tinue to mature in the adults (Pitnick and Miller, 2000). The

results of the present work indicate that a similar process

occurs in Z. indianus, because one-day-old males already

exhibited significant quantities of sperm bundles, whereas

in Z. sepsoides such quantities are were only found in older

males (8 days old).

These results contradict those of Pitnick and Miller

(2000), who showed that in D. hydei, apparently selected

for larger testes, there is an increase in development time

(egg to adult) and time of maturation of spermatozoa post-

hatching, and this may contributes to a delay in reproduc-

tion of this population. In Z. indianus, which has larger tes-

tes and sperm than Z. sepsoides, this does not occur, be-

cause young males already present considerable amounts of

mature sperm (Madi-Ravazzi, L.; David J., “personal com-

munication”)

The fertility of Z. indianus is greater than that of Z.

sepsoides, at least in laboratory experiments (Madi-Ra-

vazzi, L.; David J., “personal communication”), indicating

a difference in the fitness of these species. If the size differ-

ences and maturation of sperm from these species favor

greater reproductive potential, this phenomenon should be

investigated in detail.

The organized distribution of spermatogenic cells ob-

served in Z. indianus and Z. sepsoides is similar to that of

other Drosophilidae (Cooper, 1950; Bairati, 1968; Meyer

and Henning, 1974; Hardy et al., 1979; Lindsley and To-

kuyasu, 1980; Fuller, 1993, Joly and Bressac, 1994;

Gönczy and Dinardo, 1996, Li and Xie, 2005; Scharer et

al., 2008; Mojica et al., 2000; Barreau et al., 2008, Cheng

and Mruk, 2010).

Another relevant observation was the constant num-

ber of 64 sperm per bundle in both Z. indianus and Z.

sepsoides. The mechanism underlying the formation of

these bundles is similar to what occurs in Drosophila and

other insects. In these species, a bundle of sperm is pro-

duced by a series of mitotic divisions of the spermatogonia

within a spermatic cyst (Cooper, 1950; Hardy et al., 1979;

Lindsley and Tokuyasu, 1980; Fuller, 1993; Fabrizio et al.,

1998; Mojica et al., 2000).

Although the mitotic divisions within the cysts are

synchronous in most insects, in some species of Drosophila

the divisions can also be asynchronous and the number of

sperm per bundle may vary from 32, as in D. hydei, to 128,

as in D. pseudoobscura (Mojica et al., 2000). Similar ob-

servations were made in bees (Cruz-Landim, 2001) and

Coleoptera (Name et al., 2007).

According to Virkii (1969), basal orders of insects

have more sperm per bundle than the more derived orders,

and the more specialized groups tend to exhibit the lowest

number of sperm per bundle. Name et al. (2007) demon-

strated, for example, that certain scarab beetles have

128-512 sperm per bundle and Sitophilus zeamais and S.

oryzae approximately 260 sperm per bundle. Fewer sperm

per bundle indicates a reduced production of spermatozoa,

which may correspond to a fitness that limits genetic vari-

ability (Mojica et al., 2000). Although Z. indianus and Z.

sepsoides have the same number of sperm per bundle (64),

there may be different numbers of cysts in these species,

which is indicated by a greater abundance of sperm bundles

in Z. indianus than in Z. sepsoides.

The present study confirmed the organizational pat-

tern of the axoneme of insects following the 9+9+2 scheme,

which is an arrangement of an internal 9+2 microtubules

surrounded by nine additional accessory microtubule.

Studies of the structural organization of the axoneme in

Drosophila have been performed by various researchers

(Perotti, 1969; Kiefer, 1970; Phillips, 1970; Dallai and

Afzelius, 1991; Dallai et al., 1993; Fabrizio et al., 1998;

Mojica et al., 2000), who all confirmed the conservation of

this structure in different species of Drosophila and the

family Drosophilidae. Two mitochondrial derivatives of

different size are also present in both Zapronius species. In

insects, these organelles have been studied ultrastructurally

using a phylogenetic approach and this morphology of mi-

tochondrial derivatives is observed in highy evolved spe-

cies, being a apomorphic character (Phillips, 1970; Mojica

et al., 2000). Some functions of mitochondrial derivatives

may be related to the process of storing and releasing en-

ergy to power the mobility of the flagellum (Phillips, 1970,

Lindsley and Tokuyasu, 1980).

Phylogenetic relationships of the genus Zaprionus

has been extensively debated (Yassin et al., 2007, 2008,

2010, Yassin and David, 2010) and current reports agree

that Zaprionus is more closely related to Drosophila than

to the Sophopora subgenera, to which the melanogaster

group belongs (Russo et al., 1995; Kwiatowski and Ayala,

1999; Da Lage et al., 2007; Commar et al., 2012). The

ultrastructural morphology of the mitochondrial deriva-

tives present in Zaprionus is very similar to those found in

D. melanogaster. However, based on the above-cited

studies, such similarity is possibly a homoplasy and not

informative regarding the phylogenetic relationships of

these taxa.

The findings presented here indicate differences in

sperm maturation between the species analyzed. These dif-

ferences may favor the reproductive success and fitness of

Z. indianus and thus also relate to the invasive potential of

the species.

Rego et al. 57

Acknowledgments

We thank Prof. Dr. Jean David (CNRS, Gif-Sur-

Yvette, France) for donating the Z. sepsoides strain, and

Prof. Dr. Hermione E. Melara de C. Bicudo for valuable

suggestions. Technical assistance for TEM was provided

by José Augusto Maulim (Microscopy Center, Faculdade

de Medicina de Ribeirão Preto, Universidade de São Paulo,

Ribeirão Preto, SP). Financial support by the Coordenação

de Aperfeiçoamento de Pessoal de Nível Superior

(CAPES) is also acknowledged.

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Associate Editor: Carlos R. Machado

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60 Spermatogenesis in Zaprionus sp.